Method and system for closing plate take-over in immersion lithography

ABSTRACT

Aspects of the present disclosure provide a method and a system for closing plate take-over in immersion lithography. A plate holder is provided for a closing plate in a wafer holder of an immersion lithography system. An optical detector is provided below the plate holder for determining whether a light signal passing through the closing plate is aligned with the plate holder using the optical detector. If the light signal is aligned, a fluid containment mechanism is lowered, and the closing plate is placed into the plate holder. Alternatively, the fluid containment mechanism is lowered to surface of the closing plate, the closing plate is affixed to the mechanism, and the mechanism is raised with the closing plate. If the light signal is not aligned, an error is triggered and the scanner is stopped.

BACKGROUND

Immersion lithography is a resolution enhancement technique for exposingimages on a substrate such as a surface of a semiconductor wafer.Immersion lithography interposes a high refractive index fluid, such aswater, between a projection lens of a lithography system and thesubstrate. The substrate is typically positioned on a stage or holder,and the fluid is maintained between the projection lens and the stage bya fluid containment mechanism, e.g., a set of “shower heads.” An imagecan then be projected, such as with deep ultra-violet (DUV) radiation,through the projection lens, through the immersion fluid, and then beexposed onto the substrate surface.

In order to isolate the fluid from the substrate surface during loadingand unloading, a fluid closing plate may be used. During exposure, thefluid closing plate is placed in a plate holder of the stage. Prior towafer unloading, the fluid closing plate is lifted from the plate holderand affixed to the fluid containment mechanism. In this way, the fluidis isolated from the substrate surface while the substrate is unloading.After a new substrate is loaded onto the stage, the fluid closing plateis resituated in the plate holder. This allows the immersion fluid tore-interpose between the projection lens and the substrate surface. Theprocess in which the fluid closing plate is dislocated from the plateholder and resituated is referred to as “closing plate take-over.”

Currently, the position of the fluid closing plate is calibrated after adefined number of cycles using a transmission image sensor (TIS). Thetransmission image sensor is mounted inside the substrate holder at adistance from the plate holder. The transmission image sensor calibratesthe plate position by detecting the DUV radiation projected from theprojection lens and adjusting the plate position based on the distancebetween the detected radiation and the plate holder.

However, this type of calibration is not efficient because extramaintenance time is needed to adjust the plate position if it is out ofspecification. In addition, the calibration does not monitor plateposition after every closing plate take over. This may result in acollision of the fluid closing plate with the wafer holder or othermaterials, which may lead to damage of parts, unwanted particles, andmachine downtime.

A need exists for a method and system that monitor the position of thefluid closing plate after every closing plate take-over. In this way,collision with the wafer holder or other materials may be avoided andefficiency of immersion lithography may be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIGS. 1( a)-1(i) are diagrams illustrating a fluid closing platetake-over process according to one embodiment of the present disclosure.

FIG. 2 is a diagram of a system for closing plate take-over inaccordance with another embodiment of the present disclosure.

FIG. 3 is a diagram of a top view of a fluid closing plate used in theembodiment of FIG. 2.

FIGS. 4( a)-4(h) are diagrams of a fluid closing plate take over processin accordance with one embodiment of the present disclosure.

FIG. 5 is a flow diagram of an exemplary process for fluid closing platetake-over during wafer unloading.

FIG. 6 is a flow diagram of an exemplary process for fluid closing platetake-over during wafer loading.

DETAILED DESCRIPTION

The present disclosure relates generally to immersion lithography, suchas can be used for fabrication of semiconductor devices. It isunderstood, however, that specific embodiments are provided as examplesto teach the broader inventive concept, and one of ordinary skill in theart can easily apply the teachings of the present disclosure to othermethods and systems. Also, it is understood that the methods and systemsdiscussed in the present disclosure include some conventional structuresand/or steps. Since these structures and steps are well known in theart, they will only be discussed in a general level of detail.Furthermore, reference numbers are repeated throughout the drawings forthe sake of convenience and clarity, and such repetition does notindicate any required combination of features or steps throughout thedrawings.

FIGS. 1( a)-1(i) illustrate one embodiment of a fluid closing platetake-over process using an immersion lithography system 10. In FIG. 1(a), the immersion lithography system 10 includes a scanner mechanism 11and a wafer holder 12. The scanner mechanism 11 includes a projectionlens 13 and a fluid containment mechanism (e.g., a set of shower heads)14. Wafer holder 12 holds a wafer 15 for exposure by the projection lens13. The scanner mechanism 11 is mostly fixed in position while the waferholder 12 is easily movable to allow scanning of the wafer 15. Duringexposure, a deep ultraviolet light 16 is projected onto the wafer 15through the projection lens 13 to pattern the wafer 15. The ultravioletlight 16 may be a 193 nm UV laser.

A high refractive index immersion fluid 17 is interposed between theprojection lens 13 and the surface of wafer 15 during the exposureprocess. The set of shower heads 14, also referred to as a fluidcontainment mechanism, constrain the immersion fluid 17 underneath theprojection lens 13. The high refractive index immersion fluid 17provides a resolution enhancement, because the projection lens 13 may bedesigned with numerical aperture greater than one. A fluid closing plate18 is provided and is placed in plate holder 19 of the wafer holder 12during exposure. Prior to wafer unloading, the fluid closing plate 18 islifted from the plate holder 19 and affixed to the fluid containmentmechanism. The fluid closing plate 18 isolates the fluid from thesubstrate surface 15 while the substrate is unloading. After a newsubstrate is loaded onto the stage, the fluid closing plate 18 isresituated in the plate holder 19, which allows the immersion fluid 17to re-interpose between the projection lens 13 and the substrate surface15. The process in which the fluid closing plate 18 is dislocated fromthe plate holder 19 and resituated is referred to as “closing platetake-over.”

In one embodiment, the fluid closing plate may be made of quartz orother non-porous material. An example of the thickness of the fluidclosing plate 18 may be about 1 mm. A transmission image sensor (TIS) 20is also provided and placed within the wafer holder 12 distant from theplate holder 19. During exposure, the scanner mechanism 11 scans acrossthe surface of the wafer 15 in a wafer loading direction 27.

In FIG. 1( b), after the scanner mechanism 11 finishes scanning thesurface of wafer 15, the wafer holder 12 moves to a position where thescanner mechanism 11 is directly over the fluid closing plate 18, whichis placed in the plate holder 19. In FIG. 1( c), in order to isolate theimmersion fluid 17 from the wafer surface while the wafer 15 isunloading, the set of shower heads 14 is lowered to make contact withthe surface of the fluid closing plate 18. The fluid closing plate 18 isthen affixed to the set of shower heads 14. The fluid closing plate 18may be affixed by a vacuum force provided by the scanner mechanism 11.In FIG. 1( d), the scanner mechanism 11 is raised along with the fluidclosing plate 18, which prevents immersion fluid 17 from dispensing ontothe wafer surface. Once the fluid closing plate 18 is raised, the plateholder 19 is now empty. In FIG. 1( e), the wafer holder 12 moves in awafer unloading direction 28 and the wafer 15 is unloaded.

In FIG. 1( f), a new wafer 21 is loaded into wafer holder 12 and thewafer holder 12 moves in a wafer loading direction 27. In FIG. 1( g),the wafer holder 12 moves to a position where the scanner mechanism 11with the fluid closing plate 18 affixed is directly over the plateholder 19. In FIG. 1( h), the fluid closing plate 18 is lowered into theplate holder 19. The position of the fluid closing plate 18 isdetermined by the transmission image sensor 20, which calibrates theplate position after a defined number of wafer cycles, for example,every 400 wafers. In FIG. 1( i), once the fluid closing plate 18 is inthe plate holder 19, the fluid closing plate take-over processterminates and immersion fluid 17 is re-interposed between theprojection lens 13 and the surface of the wafer 21. The scannermechanism 11 continues to scan the wafer surface as the wafer holder 12moves in the wafer loading direction 27.

Referring now to FIG. 2, in another embodiment, an immersion lithographysystem 22 includes many of the same components of the system 10discussed above, with modifications and additional components asdiscussed below. The immersion lithography system 22 includes thescanner mechanism 11 and the wafer holder 12. The scanner mechanism 11includes the projection lens 13 and the set of shower heads 14. Thewafer holder 12 holds the wafer 15 for exposure by the projection lens13. The scanner mechanism 11 is mostly fixed in position while the waferholder 12 is easily movable to allow scanning of the wafer 15. Duringexposure, a deep ultraviolet light 16 is projected onto the wafer 15through the projection lens 13. A high refractive index immersion fluid17 is interposed between the projection lens 13 and the surface of wafer15 during the exposure process. The set of shower heads 14, alsoreferred to as a fluid containment mechanism, constrain the immersionfluid 17 underneath the projection lens 13.

The fluid closing plate 18 is provided and placed in the plate holder 19of the wafer holder 12 during exposure. Prior to wafer unloading, thefluid closing plate 18 is lifted from the plate holder 19 and affixed tothe fluid containment mechanism. The fluid closing plate 18 isolates thefluid from the substrate surface 15 while the substrate is unloading.After a new substrate is loaded onto the stage, the fluid closing plate18 is resituated in the plate holder 19, which allows the immersionfluid 17 to re-interpose between the projection lens 13 and thesubstrate surface 15. The process in which the fluid closing plate 18 isdislocated from the plate holder 19 and resituated is referred to as“closing plate take-over.”

In one embodiment, the fluid closing plate 18 may be made of quartz orother non-porous material. An example of the thickness of the fluidclosing plate 18 may be about 1 mm. The transmission image sensor 20 isalso provided and placed within the wafer holder 12 distant from theplate holder 19.

In addition, a position alignment system including an optical detector23 is provided within wafer holder 12. Those of ordinary skill in theart will know of many different types of optical detectors that can beused. The optical detector 23 is located below the fluid closing plate18 situated in the plate holder 19. The optical detector 23 ispreferably located below the center of the plate holder 19 with equaldistance from both ends of plate holder 19. In an illustrativeembodiment, the optical detector 23 detects the plate position byanalyzing the deep ultraviolet light 16 passing passes through a patternof the fluid closing plate 18. If center of the deep ultraviolet light16 is aligned with the center of the plate holder 19, the position ofthe fluid closing plate 18 is aligned with the plate holder 19 and thefluid closing plate 18 may be lowered to the plate holder 19. Otherwise,an error is triggered to indicate that the position of the fluid closingplate 18 is shifted out of specification.

FIG. 3 is a diagram of a top view of the fluid closing plate accordingto the embodiment discussed above in FIG. 2. In this embodiment, thefluid closing plate 18 includes a circular pattern 24. When the waferholder 12 moves to a position where the scanner mechanism 11 is directlyover the fluid closing plate 18, the optical detector 23 analyzes anddetermines if the center of deep ultraviolet light 16 passing throughpattern 24 is aligned with the center of the plate holder 19. The centerof deep ultraviolet light 16 is aligned with the center of the plateholder 19 if the center of deep ultraviolet light 16 lies exactly in thecenter of the plate holder 19. If the optical detector 23 detects thatthe center of deep ultraviolet light 16 does not align with the centerof the plate holder 19, the position of the fluid closing plate 18 isshifted out of specification. At this time, an error indicator may betriggered and the operation of the scanner mechanism 11 may be halted.Appropriate correction can then be initiated to rectify the problem.

FIGS. 4( a)-4(c) are diagrams of a fluid closing plate take over processin accordance with one embodiment of the present disclosure using theimmersion lithography system 22 of FIGS. 2 and 3. In FIG. 4( a), afterthe scanner mechanism 11 finishes scanning the surface of wafer 21, thewafer holder 12 moves to a position where the scanner mechanism 11 isdirectly over the fluid closing plate 18 in the plate holder 19. At thistime, the optical detector 23 analyzes and determines whether the centerof deep ultraviolet light 16 passing through the circular pattern 24 isaligned with the center of the plate holder 19. The center of deepultraviolet light 16 is aligned with the center of the plate holder 19if the center of deep ultraviolet light 16 lies exactly in the center ofthe plate holder 19.

If the center of the deep ultraviolet light 16 does not align with thecenter of the plate holder 19, an error is triggered to indicate thatthe position of the fluid closing plate 18 is out of specification andthe scanner mechanism 11 is stopped. In FIG. 4( b), if the center of thedeep ultraviolet light 16 is aligned with the center of the plate holder19, the set of shower heads 14 is lowered to the surface of the fluidclosing plate 18 and the fluid closing plate 18 is affixed to the set ofshower heads 14. The fluid closing plate may be affixed by a vacuumforce provided by the scanner mechanism 11.

In FIG. 4( c), the scanner mechanism 11 is raised along with the fluidclosing plate 18, which isolates immersion fluid 17 from the wafersurface. Once the fluid closing plate 18 is raised, the plate holder 19is now empty. In FIG. 4( d), the wafer holder 12 moves in a waferunloading direction 28 and the wafer 21 is unloaded. In FIG. 4( e), anew wafer 26 is loaded onto wafer holder 12 and the wafer holder 12moves in a wafer loading direction 22. In FIG. 4( f), the wafer holder12 moves to a position, where scanner mechanism 11 with the fluidclosing plate 18 affixed is directly over the plate holder 19. Beforethe fluid closing plate 18 is lowered into the plate holder 19, opticaldetector 23 analyzes and determines whether the center of the deepultraviolet light 16 passing through pattern 24 is aligned with thecenter of the plate holder 19. The center of deep ultraviolet light 16is aligned with the center of the plate holder 19 if the center of deepultraviolet light 16 lies exactly in the center of the plate holder 19.If the center of deep ultraviolet light 16 does not align with thecenter of the plate holder 19, an error is triggered and the scannermechanism 11 is stopped.

In FIG. 4( g), if the optical detector 23 detects that the center of thedeep ultraviolet light 16 aligns with center of the plate holder 19, theset of shower heads 14 is lowered and the fluid closing plate 18 isplaced into the plate holder 19. In FIG. 4( h), once the fluid closingplate 18 is in the plate holder 19, the fluid closing plate take-overprocess terminates and immersion fluid 17 is re-interposed between theprojection lens 13 and the surface of the wafer 26. The scannermechanism 11 continues to scan the wafer surface as the wafer holder 12moves in the wafer loading direction 27.

FIG. 5 is a flow diagram of an exemplary process for fluid closing platetake-over for wafer unloading. The take-over process begins at step 30when the wafer holder moves to a position where the scanner is directlyover the plate holder after exposure. Next, a determination is then madeat step 32 as to whether the center of the deep ultraviolet lightpassing through the circular pattern of the fluid closing plate isaligned with the center of the plate holder 19. The center of deepultraviolet light 16 is aligned with the center of the plate holder 19if the center of deep ultraviolet light 16 lies exactly in the center ofthe plate holder 19. If the center of the deep ultraviolet light doesnot align with the center of the plate holder 19, an error is triggeredand the scanner is stopped at step 34. The take-over process terminatesthereafter.

If the optical detector detects the center of the deep ultraviolet lightaligns with the center of the plate holder 19, a set of shower heads islowered to the closing plate surface at step 36. At step 38, the closingplate is affixed to the set of shower heads. The set of shower heads isthen raised with the closing plate at step 40 and the wafer is unloadedat step 42.

FIG. 6 is a flow diagram of an exemplary process for fluid closing platetake-over for wafer loading. The take-over process begins at step 44when the wafer holder moves to a position where the scanner is directlyover the plate holder after a new wafer is loaded. A determination isthen made at step 46 as to whether the center of the deep ultravioletlight passing through the pattern is aligned with the center of theplate holder. If the center of the deep ultraviolet light does not alignwith the center of the plate holder, an error is triggered and thescanner is stopped at step 48. The take-over process terminatesthereafter.

If the optical detector detects that the center of the deep ultravioletlight aligns with the center of the plate holder 19, the set of showerheads is lowered at step 50. At step 52, the closing plate is placed inthe plate holder. The immersion fluid is re-interposed between theprojection lens and the wafer surface at step 54. The scanner thencontinues to scan the wafer at step 56.

In summary, aspects of the present disclosure provide a method andsystem for fluid closing plate take-over in immersion lithography. Byproviding an optical detector under the plate holder of the waferholder, the position of the closing plate may be monitored inline afterevery take-over. The position of the closing plate may be monitoredbased on the center of the deep ultraviolet light relative to the centerof the plate holder. In addition, collision of the closing plate and thewafer holder or other materials may be avoided. Furthermore, efficiencymay be improved as the need for calibration by the transmission imagesensor (TIS) 20 is eliminated.

It is noted that the optical detector may also be implemented withoutthe TIS. In that case, the optical detector may perform calibrationfunction similar to TIS, but in a more accurate fashion with the abilityof collecting plate position data after every take-over. In addition,the optical detector may be implemented such that the movement of theclosing plate may be traced as the scanner is scanning the wafer. Inthis way, closing plate take-over may be safely performed each timewithout damaging the wafer or other materials.

Although embodiments of the present disclosure have been described indetail, those skilled in the art should understand that they may makevarious changes, substitutions and alterations herein without departingfrom the spirit and scope of the present disclosure. Accordingly, allsuch changes, substitutions and alterations are intended to be includedwithin the scope of the present disclosure as defined in the followingclaims. In the claims, means-plus-function clauses are intended to coverthe structures described herein as performing the recited function andnot only structural equivalents, but also equivalent structures.

1. An immersion lithography system comprising: a scanner for exposing a semiconductor wafer, wherein the scanner comprises: a projection lens operable to project a light signal; an immersion fluid interposing between the projection lens and surface of the wafer; and a set of shower heads operable to constrain the immersion fluid underneath the projection lens; and a wafer holder for holding the semiconductor wafer for exposure by the scanner, the wafer holder comprising a plate holder for holding a closing plate, and an optical detector, wherein the optical detector is located below the plate holder, whereby the scanner is operable to stop operations if the light signal is not aligned with the plate holder.
 2. The system of claim 1, wherein the optical detector is located below a center of the plate holder with equal distance from both ends of the plate holder.
 3. The system of claim 1, wherein the optical detector is operable to determine whether a light signal passing through the closing plate is aligned with the plate holder.
 4. The system of claim 1, wherein the scanner is operable to place the closing plate into the plate holder if the light signal is aligned with the plate holder.
 5. The system of claim 1, wherein the scanner is operable to affix the closing plate to a set of shower heads if the light signal is aligned with the plate holder.
 6. A method of performing immersion lithography, the method comprising: providing a scanner for exposing a semiconductor wafer; projecting a light signal through a projection lens; interposing an immersion fluid between the projection lens and a surface of the wafer; constraining the immersion fluid underneath the projection lens using a set of shower heads; holding the semiconductor wafer using a wafer holder for exposure by the scanner, wherein the wafer holder comprises a plate holder for holding a closing plate and an optical detector, wherein the optical detector is located below the plate holder; and stopping operations by the scanner if the light signal is not aligned with the plate holder.
 7. The method of claim 6, further comprising determining whether a light signal passing through the closing plate is aligned with the plate holder using the optical detector.
 8. The method of claim 6, further comprising placing the closing plate into the plate holder using the scanner if the light signal is aligned with the plate holder.
 9. The method of claim 6, further comprising affixing the closing plate to a set of shower heads using the scanner if the light signal is aligned with the plate holder. 